This invention relates to silicon based reflective liquid crystal displays (LCDs) and projection devices comprising a light source, such LCDS having a layer of liquid crystalline material between a first and second substrate which may be provided with orientation means, preferably the molecules of the liquid crystalline material having a twist angle xcfx86, means for guiding light from the light source to the first substrate, the second substrate being provided with means for reflecting light passing the layer of liquid crystalline material, said projection device further comprising polarizing means in the light path between the light source and the first substrate and analyzing means in the light path, after reflection, between the liquid crystal display device and a display plane.
Silicon based reflective LCDs are potentially the most cost effective solution for high performance, high resolution digital projection in both the business and consumer markets. To maintain this cost effectiveness, a single panel approach is desirable. Reflection offers an advantage to this approach because LCDs can be made thinner and faster, thus offering the brightness and color quality expected in the high performance market via the single panel approach. LC effects which can switch to dark as fast as possible are desirable for single panel operation because they can block the unwanted colors during scanning most effectively with minimal use of dark regions between the scanning colors (guard bands). This results in high color purity and higher brightness when compared with slower responding devices, such as transmissive LCDS. The physical properties of liquid crystals imply that the electric field applied transition should be to a dark state in order to meet this requirement. Thus, optimally, all candidate effects should be xe2x80x9cnormally whitexe2x80x9d, i.e., fully transmissive with no field applied. This means that with the highest voltage on the electrodes, no light is directed to the viewer from these areas/pixels and therefore they appear dark.
Normally white LC effects can be separated into two different categories: those requiring external foil compensation and those which do not require external foil compensation. A disadvantage of both categories, however, is that areas of inactivity (passive regions where no voltage can be applied) in the border of the display device appear bright.
Furthermore, it may be desirable to integrate driving electronics into the silicon immediately outside the active area. These areas must be shielded from light in order to function properly. A simple layer of aluminum above these circuits would most effectively shield them from light but this layer would act as a mirror reflecting most of the incident flux directly to the viewer with a xe2x80x9cnormally whitexe2x80x9d LC effect. Certain precautions have been recommended to render these areas dark to the viewer, for example, by replacing the aluminum with a black chromium layer. This requires an extra masking step which makes the display more expensive, while it also adds difficulty to the process of coupling the two substrates. Furthermore, chromium is not a commonly used material in silicon processing as it can render circuitry inoperable due to contamination.
Such a light absorbing mask could be used on the passive plate, but then it must be critically aligned to the active region of the silicon chip. Additionally, in order to absorb sufficiently, a minimum thickness of material is required and as such will have to be compensated for in order to maintain uniformity in a thin cell gap. Furthermore, the light energy absorbed will be converted to a significant amount of heat under the intensities expected in any projection system.
There is a need in the art for rendering these undriven display regions dark with a normally white LC effect in which these disadvantages are obviated as much as possible while still effectively shielding any underlying circuitry from the incident light.
An object of this invention is to provide a display device which uses a normally white LC effect that is free of or in which the above-described disadvantages are obviated as much as possible.
Another object of the invention is to make the regions that integrate driving electronics into the silicon immediately outside the active area appear dark to a viewer while still offering effective light shielding for the underlying circuitry.
Another object of the invention is to provide a display device, for use in reflection, that is provided with a border structure around an active display area, said border being effective to shield areas of said display device from incident light and to render said shielded areas dark to a viewer in the non-driven state.
These and other objects of the invention are accomplished by the provision of a projection display device, according to one embodiment of the invention, which comprises a layer of liquid crystalline material between a first and a second substrate, which substrates are optionally and preferably provided with orientation means and the molecules of the liquid crystalline material optionally and preferably having a twist angle xcfx86, means for guiding light from the light source to the first substrate, the second substrate being provided with means for reflecting light passing the layer of liquid crystalline material, preferably a pixel array active area of said substrate, said projection device further comprising polarizing means in the light path between the light source and the first substrate and, optionally and preferably, analyzing means in the light path, after reflection, between the liquid crystal display device and a display plane,
wherein a portion of the liquid crystalline material is displaced by a dam comprising a substantially transparent, non-birefringent material such as SiNx, said dam forming a border, preferably, a substantially continuous border which substantially surrounds the pixel array active area of the display device, and wherein, most preferably, the liquid crystal display device is provided with a retardation foil or other compensation device, substantially all areas of said display device being dark in a non-driven state without absorbing substantial energy of the incident light flux.
As used herein, the phrase xe2x80x9cnon-driven statexe2x80x9d means any voltage below the threshold for reorientation of the liquid crystalline material that is applied across the cell gap.
In another embodiment of the invention, there is provided a projection display device which comprises at least one light source whose light is incident on a display device as described hereinabove and is reflected dependent on the optical state of pixels defined by picture electrodes surrounded by a substantially continuous border comprising a dam of non-birefringent material, the light thus modulated being imaged by projection means, the areas situated between the electrodes and also other non-switching areas, for example, electronics integrated along the edge of the picture sections of said display device, being effectively shielded from the incident light yet appearing substantially dark in the non-driven state.
Thus, the invention involves the concept of a dark border employing a non-birefringent material on the active plate, around the active matrix of the reflective LC display device, and the use of LCDs comprising such borders. The border may be formed from a xe2x80x9cdamxe2x80x9d of any non-birefringent material, preferably a photodefinable spacer material already patterned or otherwise provided on the active plate. Preferably, the thickness of the dam is such as to allow a thin layer of LC material on top of the dam, and most preferably, the border structure shields the areas of underlying circuitry between the glue seal and the active region.
By design, this border has proven effective for both foil compensated and non-foil compensated effects.
As stated hereinabove, the invention is particularly suitable for devices using a xe2x80x9cnormally white LC modexe2x80x9d in which, in the non-driven state, not only the picture electrodes reflect light but also the structures and light shield material situated between and around the picture electrodes, so that extra steps have heretofore been required to render this interpositioned material invisible. Such normally white LC modes can usually be separated into two different categories: those requiring external foil compensation and those which do not require external foil compensation. The following Table lists some of these display devices;
For purposes of discussion and illustration of the invention, the invention will be illustrated in terms of the normally white device 45TN0, however any and all of the above devices may be used in this invention.
For example, such effects and devices may be used as described in commonly assigned U.S. Pat. No. 5,490,003 of Van Sprang, issued Feb. 6, 1996. According to the Sprang patent, a substantial extinction of the light is obtained in the dark state by controlling the orientation directions of the LC alignment layers (orientation means) with respect to the direction of polarization of the polarizing means (the direction of the polarization is along the direction of the bisecting line between the LC orientation directions). A range of twist angles between 50 and 68 degrees and a product (d.xcex94n) of the thickness d of the layer of liquid crystal material and the birefringence xcex94n of the liquid crystal material within the range of 0.58 xcex0 and 0.68 xcex0, in which xcex0 is a central wavelength, can be used for a good contrast. Additionally, the difference in optical path length, after reflection, between the ordinary and the extraordinary wave is approximately 0.5 xcex0 for the two extreme transmission states and the reflective liquid crystal display device functions as an electrically controllable quarter-lambda plate.
Other devices as described in commonly assigned EPO 97201795.8, published December, 1998 have been found to be especially useful in this invention. According to EPO 97201795.8, the reflective display device uses a twist angle having a value between 30 and 55 degrees and a retardation foil, which makes it possible to reduce the drive voltage while maintaining high contrast. The xe2x80x9cresidual transmissionxe2x80x9d in a fully driven display element is compensated by the retardation value of the foil, which permits a smaller thickness d so that the device switches more rapidly. By choosing the orientation direction of the liquid crystal molecules on the side of the polarizing means to be substantially parallel to the direction of polarization of the incident light, or by choosing this orientation direction to be perpendicular to the direction of polarization, it is only necessary to compensate for the birefringence due to a non-reorientable surface layer of LC on the side of the reflecting means. Also, the angle xcex between the optical axis of the retardation foil and the orientation direction of the orientation means at the area of the first substrate may vary between, for example, 80xc2x1xcfx86/2 degrees and 100xc2x1xcfx86 degrees.
The disclosures of said U.S. Pat. No. 5,490,003 and EPO 97201795.8 are hereby incorporated into the present disclosure by this reference thereto.
As discussed hereinabove, when a normally white LCD is rendered dark, there are areas of inactivity (xe2x80x9cno voltage appliedxe2x80x9d) in the border regions of the display which remain white. The present invention in its most preferred embodiments provides a border structure comprising a dam around the active display area of the LCD to render areas of inactivity in the border area dark at all times. Preferably, the dam is positioned between the glue seal and the active region, the thickness of the dam being such as to allow a thin layer of LC material on top of the dam. In such case, the border structure is also effective to keep the glue from seeping into the active region when the seal line is squeezed during assembly. With a high birefringence LC material and an external compensation foil, a high degree of light extinction is achieved over the border. Also, formation of the border on the active plate means that no critical alignment of the passive plate is needed.
The invention thus provides a display device which uses a normally white LC effect and a method for rendering non-driven regions of a normally white LC effect dark and which is characterized by the following advantages:
1. a high degree of light extinction;
2. low to no light absorption;
3. a critical alignment of the passive plate is not required;
4. no additional cell processing steps are required to achieve a dark border;
5. a highly effective ionic contamination barrier between the active region and the edge seal region of the LCD is provided; and
6. the LC in said region cannot be reoriented by electric fields normally applied which would reduce the extinction of said border.